This proposal is based on recent experimental results with paired congenic and transgenic strains of mice with divergent missing-self MHC-I Dk recognition of murine CMV infected cells to examine the role of NK cells in virus infected animals. The broad long-term objective for the research project seeks to understand inhibitory receptor signaling in NK cells and the impact on the NK cell's ability to rapidly respond to and kill virus infected cells. An interesting question raised by the research proposal is whether NK inhibitory receptor recognition of virus infection simply permits activation receptor signals in NK cells or directly activates a signal cascade in NK cells needed in efficient effector responses and viral resistance. This distinction is important since NK cells have a profound effect on other immune cells, including the priming of virus-specific T cell effectors. A guiding hypothesis in the research proposal is that NK inhibitory receptor recognition of virus infection delivers a potent signal which permits or 'activates'rapid NK responses to immediately provide protection against further infection and begin to cultivate other immune cells needed to prime adaptive immune responses. Three specific aims are proposed: Aim 1. will determine the effect of inhibitory receptor signaling and missing-self MHC class I recognition of virus infection in NK cell-mediated resistance and enhancement of adaptive viral immunity. We will test the hypothesis that an inhibitory receptor is required in missing-self MHC-I Dk recognition of MCMV infected cells in Ly49G-null mice. Proposed experiments will examine the effect of Ly49G2 deficiency on innate virus resistance, priming of virus-specific T cell effectors, viral persistence and mortality. Aim 2. will assess the effect of inhibitory receptor signaling and missing-self recognition of virus infected target cells on membrane proximal signaling events in NK cells and identify key NK cell expression differences, which may be causally linked with innate virus resistance. We will investigate Ly49G2 inhibitory receptor signaling in the context of virus infection and the hypothesis that missing-self recognition of MHC-I Dk infected cells permits or activates a potent signal, marked by distinct biochemical changes in NK cells, to promote NK cell responsiveness. Aim 3. will perform refined genetic mapping, identification and characterization of non-MHC loci that enhance inhibitory receptor expression on NK cells and NK cell-mediated virus resistance. We will test the hypothesis that modifiers of Ly49G2 receptor expression and MCMV resistance determine how NK cells respond to infection, their capacity to recognize infected cells and consequently their impact on adaptive immunity. Relevance NK cells are vital to human health. They protect against malignancy and virus infection. NK cells recognize and respond to many different types of viruses. However, some viruses are able to evade NK cell detection (e.g. herpesviruses) and others (e.g. HIV) are able to outlast, outwit or defeat NK cells. Thus, it is critical to understand how NK cells recognize virus infected cells in the body. Recent advances in this field have remarkable promise since we know now that NK cells can use activation receptors that directly recognize virus infected cells and therefore target NK cell killing adequately. However, NK cells with activation receptors cannot restrain all virus infections. Despite this, NK cells use inhibitory receptors also to recognize and respond to viral infection. Unfortunately, an established model to investigate the effect of NK inhibitory receptors in viral infections was not available before. Our laboratory has succeeded in generating such an innovative model and this research proposal will use it to investigate the impact of NK cell inhibitory receptors and their contribution to antiviral NK cell effector functions.